1. Field of the Invention
The present invention relates to a radiation image storage panel employed in a radiation image recording and reproducing method utilizing a stimulable phosphor, and a process for the preparation of said panel.
2. Description of the Prior Art
For obtaining a radiation image, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance in U.S. Pat. No. 4,239,968, has been proposed and practically used. In the method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is employed, and the method involves steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals on a recording material such as a photographic film or a display device such as CRT.
According to this method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to an object at considerably smaller dose, as compared with a conventional radiography employing a combination of a radiographic film and a radiographic intensifying screen. The method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the above-described method has a basic structure comprising a support and a phosphor layer provided on one surface of the support. When the phosphor layer is self-supporting, the support is not always necessary. Further, a transparent film of a polymer material is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The phosphor layer generally comprises a binder and a stimulable phosphor dispersed therein. The stimulable phosphor emits light (gives stimulated emission) when excited with an electromagnetic wave (stimulating rays) such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
The radiation image recording and reproducing method is very useful for obtaining a radiation image as a visible image as described hereinbefore. It is desired for the radiation image storage panel employed in the method to have a high sensitivity and provide an image of high quality (high sharpness, high graininess, etc.), as well as the radiographic intensifying screen employed in the conventional radiography.
The sensitivity of the radiation image storage panel is essentially determined by the total amount of stimulated emission given by the stimulable phosphor contained therein, and the total emission amount varies depending upon not only the emission luminance of the phosphor per se but also the content of the phosphor in the phosphor layer. The large content of the phosphor also results in the increase of absorption of a radiation such as X-rays, so that the panel has a higher sensitivity and provides an image improved in quality (especially graininess). On the other hand, when the content of the phosphor is the same, the panel provides an image of high sharpness as the phosphor layer is charged densely therewith, because the phosphor layer can be made thin to reduce the spread of the stimulating rays owing to the scattering.
The phosphor layer has been usually formed by adding stimulable phosphor particles and a binder to an appropriate solvent to prepare a coating dispersion and then applying the coating dispersion onto a support or a sheet using a known coating means such as a doctor blade or a roll coater to dry. The formed phosphor layer comprising the binder and the stimulable phosphor dispersed therein has a relative density (proportion by volume of the phosphor occupying the phosphor layer) limited to approx. 60%. Further, since the phosphor layer having the binder contains a great number of air bubbles, the stimulating rays and the emitted light tend to scatter.
To increase the content of the phosphor in the phosphor layer without making the thickness thereof large (i.e., a method for increasing the relative density of the phosphor layer), there has been known a method of compressing a phosphor layer (or a phosphor layer and a support) using a compression means such as a calender roll or a hot press (U.S. patent application Ser. No. 568,416). The obtained phosphor layer has the relative density enhanced and contains more phosphor particles than the conventional one in the same thickness. However, the phosphor is strained by the compression and it is apt to lower the sensitivity of the panel (referred to as "pressure desensitizing").
As for a method for forming a phosphor layer which contains no binder but a stimulable phosphor only, there is described in U.S. Pat. No. 3,859,527 that a temporary storage medium comprises a hot pressed phosphor, and in Japanese Patent Provisional Publication No. 61(1986)-73100 (the amendment filed on Sept. 11, 1985 for Japanese Patent Application No. 59(1984)-196365) that a phosphor layer is formed by a firing process. However, both the description merely indicates that the hot press process and the firing process can be employed to form the phosphor layer.